Yes. This should be a priority. This may be as simple as a parasite problem, giardia or coccida. Or it could be a respiratory issue.
There's no doubt that you can't sell the puppy until he is well.
So, go ahead and see the vet with him as soon as you can.
This is also a good time to take stool samples from all the puppies with you so that you can get fecal exams.
Let me know if you need more help!